IDE :: Difference Between CS3 API FlashPlayer And FlashPlayer In A Browser?

I picked up enough AS3 recently to create some advertising banners and they work perfectly when I test the movie within CS3 by using ctrl+enter. I uploaded the created swf on the website and experienced lots of weird problems like child mcs stopping to play after a while or movies not playing till the end etc.. On some movies, they will play properly if you reload the website, on some others (the one which doesnt play till the end) that wont fix the problem either. The banners' filesizes are also around 8KB for example and include only text and vectors.

I published my movie by letting Flash create an html file, and the exact same problems are occurring there aswell. I mean I got strict mode and everything turned on in Flash but I'm not getting any errors at all when testing the movie. I also dont see any visual errors. It's just the swf acting differently in a browser.

So I'm trying to figure out what the difference between the flashplayer used by CS3 and the one in firefox is for example. I got the latest flashplayer (10 r22) on firefox and it seems that CS3 is using the flashplayer 9 (I'm not sure about this but its the player I found in the CS3 folder).I'm using timeline based tweens (I know it sucks and I did this mostly on the first banners I created and started using the tween classes built in Flash later on when I learned more AS3) so wondering if this is the problem.